The Virginia Department of Agriculture has announced the first two cases of West Nile virus in Virginia horses. A 6-year-old, Thoroughbred mare in Culpeper, Va., was euthanized a few days after showing symptoms of WNV, beginning on May 1. The second case, a 14-year-old Quarter Horse mare in Prince Edward County, became ill on June 1 and was also euthanized.

This is the earliest WNV has ever been detected in Virginia. In 2003 the first case didn’t appear until July 25, and in 2002 and 2001 the virus didn’t show up until August and September.

“We’ve had a steady backward progression,” said Elaine Lindholm, a spokesperson for the Virginia Department of Agriculture.

Scientists cannot say why WNV is appearing earlier each year, although t some speculate that last September’s hurricane created more standing water than usual, providing a breeding area for mosquitoes and possibly contributing to the early outbreak.

The mare in Culpeper had been vaccinated just two weeks prior to her WNV infection. The equine WNV vaccine requires two doses, administered three to six weeks apart, and isn’t affective until four to six weeks after the second dose. The mare in Prince Edward County had also not been fully vaccinated.

The virus first appeared in 1999 in New York City. Last year there were 5,181 cases of equine WNV reported in 42 states. Of those, 234 cases were in Virginia, a dramatic increase from 47 cases in 2002. Scientists believe that the virus will be endemic in all states except Hawaii and Alaska by the end of the summer. Several Canadian provinces have also experienced the virus.

There is no cure for WNV.

Elsewhere in the country, veterinary cases of WNV appeared in January in Texas (five cases) and Alabama (three cases). Arizona (three cases), Oklahoma (two cases), and Missouri (one case) began to report WNV in April and May.

Up-to-date information on the location of WNV cases can be found at http://westnilemaps.usgs.gov/index.html